Ate the Equator, measure the circumference of your world globe. Let this measurement equal 100%
Then measure the lengths of the ocean waters from each coastline to the next, going either clockwise or counterclockwise around the globe. Determine what percentage of the entire equatorial circumference this is and label your answer % ocean at Equator.
Repeat this process for the 40 degree North and 40 degree South latitudes. This will give you two different percentages labeled "% ocean at 40 degrees North Latitude and % ocean at 40 degrees South Latitude.
Repeat this process again for 60 degrees North and 60 degrees South Latitudes.
You now have percentage of ocean at 60 degrees North, 40 degrees North, 0 degrees North (Equator), 40 degrees South, and 60 degrees South.
For the Northern and Southern hemispheres, find an "equal area map" of the northern hemisphere adn cut out away everything that is not either land or water. Then weigh this map. Once you have that weight, label that weight 100% Northern hemisphere. Then cut out all of the land from your map so that you are left with only ocean areas in the northern hemisphere and weigh this part of the map, and determine what percentage of the entire map is represented by ocean.
Repeat the process above for the Southern hemisphere.
Following this procedure will give you very good approximations of the percentage of ocean at each 60, 40, and 0 latitude plus north and then south hemisphere.
I do not know of any equation that will allow you to figure percentage of ocean at a specific latitude of in a specific hemisphere, but I hope this information is helpful to you!